Use GPU where available; animated zooming

This change brings major performance improvements for dragging and pinch-zooming. It also significantly simplifies the removeBackBufferDelay logic, because removing the backbuffer is now just bound to the also delayed loadend event. And finally, removeBackBufferDelay is not needed any more, except for tweaking flickers with singleTile layers on slow mobile devices.

Edit: With 21448d2, this pull request now also includes animated zooming.